Much effort has been directed to the development of assemblies for separating blood serum or plasma from the cellular portion of the blood. To accomplish this separation the assembly, typically an elongated glass cylinder of varying design and containing the blood sample, is subjected to ultracentrifugation until the cellular portion (heavy phase) and serum or plasma (light phase) are completely separated. To maintain separation after centrifuging and to facilitate decanting or removal of the light phase by pipette a barrier means is provided within the assembly between the phases.
Numerous sealing devices manufactured from elastomeric materials have been proposed as barriers. To illustrate just a few, reference may be had to the following U.S. Pat. Nos.: 3,779,383; 3,891,553; 3,894,951; 3,897,337; 3,897,340, 3,897,343, 3,931,010; and 3,957,654. Also, U.S. Pat. No. 3,919,085 describes an elastomeric barrier which is in a deformed condition prior to separation but after centrifuging assumes a sealing configuration at the interface of the two phases. The use of encapsulated beads to establish a seal between a pair of slideable cylinders is described in U.S. Pat. No. 3,909,419. Beads or granules having the proper specific gravity and coated with an adhesive material, such as a silicone fluid, are described as barriers in U.S. Pat. No. 3,920,557. U.S. Pat. No. 3,972,812 describes the use of porous discs made of a polymeric material, such as sintered nylon, styrene, polyethylene or polypropylene, and sprayed with a hydrophobic material.
The current trend is away from the use of manufactured sealing devices requiring extremely close tolerances and which are costly and difficult to manufacture. Recent emphasis has been toward the development of liquid sealant materials which are inserted directly into the assembly before or after collection of the blood sample and which accomplish the same result. For example, the separation of a sample of blood into the serum and clot portions is accomplished using a sealant (barrier) consisting of a silicone fluid having silica dispersed therein as shown in U.S. Pat. No. 3,780,935. U.S. Pat. No. 4,018,564 describes a method for analyzing a blood sample by the addition, prior to centrifuging, of a small but effective amount of a three-component blood separating composition comprising a diorganopolysiloxane, a silica filler and polyether stabilizer. Such barrier compositions, in addition to being inert and having the proper density to enable them to locate between the two phases, are highly viscous, homogeneous, hydrophobic materials. By homogeneous is meant that the materials themselves will not separate into two or more phases upon ultracentrifugation.
It would be highly advantageous if lower cost polymeric materials, such as polyesters, could be advantageously employed in such applications. Both terminated and non-terminated polyesters and copolyesters derived from a variety of polyols and polybasic acids and having molecular weights from about 500 to 50,000 or higher are known. It is also known to employ mixtures of polyols and polybasic acids to obtain useful products. U.S. Pat. Nos. 3,055,869 and 3,194,791, for example, disclose processes for preparing polyesters from a wide variety of dicarboxylic acids and dihydroxy compounds with an optional chain-terminating monobasic acid or monohydric alcohol. U.S. Pat. No. 3,057,824 also teaches that mixed polycarboxylic acids and mixtures of diols can be esterified for the preparation of polyesters having high hydroxyl values which are useful for the preparation of polyurethane coatings and foams. Polymeric fatty acids are disclosed as suitable dicarboxylic acids in these and in other references such as U.S. Pat. No. 2,429,219 where superpolyesters having molecular weights greater than 10,000 are obtained by reacting dimer acid and linear aliphatic glycols. Adipic acid is also employed with the dimer acid for the preparation of one of the superpolyesters. U.S. Pat. No. 2,384,443 also describes rubber-like compositions obtained by reacting a polymeric fatty acid and straight-chain aliphatic glycols. Mixtures of dimer and aromatic dicarboxylic acids have been reacted with polymethylene glycols as shown in U.S. Pat. Nos. 3,235,520 and 3,383,343.